A TMR (tunnel magnetic resistance) device is a device in which a very thin insulating layer is inserted between two ferromagnetic layers. A TMR device uses the phenomenon that the tunneling current flowing through the insulating layer is changed by the relative angle of magnetization of each metal element M.
It has been expected in theory that when using a ferromagnetic metal element M having a high spin polarizability such as Fe or FeCO for the ferromagnetic layers, a high rate of change in magnetic resistance of at least 35% is obtained (M. Jullier, Phs. Lett. 54A (1975) 225). However, a high MR (magneto resistance) has not been possible to realize.
Recently, Miyazaki et al. reported that they produced an insulating layer of alumina by natural oxidation in the air, and obtained a high rate of change in MR (T. Miyazaki and N. Tezuka, J. Magn. Magn. Mater. 139 (1995) L231). With this report, active development of TMR materials and TMR devices has started.
The recently reported methods for producing insulating layers showing a high MR are classified largely into two methods. One is a natural oxidation method in which an aluminum film formed on a ferromagnetic film is oxidized in the air or in pure oxygen (Tsuge et al., Document of the 103rd Workshop by the Society of Applied Magnetics of Japan, p. 119, (1998)). The other is a plasma oxidation method in which an aluminum film formed on a ferromagnetic film is oxidized in an oxygen plasma (J. S. Moodera et al. Phy. Rev. Lett., 74, 3273 (1995)).
To obtain a high MR, these TMR devices use a transition metal showing a high spin polarizability such as Fe or CoFe for the lower ferromagnetic layer on which the aluminum film is formed.
Because the current flowing in a TMR device is mainly a tunneling current through an insulating layer, the resistance of the device is substantially high. Thus, when a TMR device is used as a reproducing head or MRAM, S/N ratio decreases due to thermal noise, and threshold frequency of a readout circuit decreases during a fast response.
To lower the resistance of the device, reducing the film thickness of the alumina insulating layer could be considered. However, with a conventional process for oxidizing an aluminum film, the lower ferromagnetic film is likely to be oxidized beyond the aluminum film when the aluminum film is thin. As a result, when antiferromagnetic materials such as Fe2O3 and CoO are formed at the interface with the aluminum oxide film by an excess oxidation reaction, for example, due to the interaction with these antiferromagnetic oxides, tunneling electrons lose information of magnetization direction with an external magnetic field.
On the other hand, when the aluminum film is not oxidized completely and a portion of the aluminum film remains, the spin memory of the tunneling electrons passing through the remaining aluminum film is lost, and MR is reduced.
Furthermore, in conventional TMR devices, when a large bias is applied, the rate of change in MR is decreased greatly due to generation of magnon, etc.
Furthermore, conventional MR devices do not have a sufficient thermal stability, and for example, when using them as MRAM, heat deterioration such as decreased MR property is caused during post-annealing of CMOS (at about 250 to 400° C.) or heating in the production of MR heads (at about 250° C.), or during its use.